I've never subscribed to the "turn your tracers off to improve your gunnery" idea. Sure, once you've learned to hit consistantly it may not matter whether you have them on or off, and I suppose I could gain 2-3 kills per month if I turned them off for "stealth" purposes. Honestly though, when I shoot at someone I simply don't care whether they know I'm shooting or not...
But for someone trying to actually improve on their hit% I think the tracers are too important to turn them off.
With them on, you can miss your target and see "ah, I missed behind him, I need to lead more the next time I have a shot like that..."
With them off, you miss your target and see "ah, I missed him! Next time I'll aim higher or lower, or lead him more or lead him less or lead him more and aim higher or lead him more and aim lower... and I might actually pick the right option and actually hit him!" Or "Maybe I did everything right, but my convergence is wrong?" Or "Maybe the only people in the game who can make a shot like that are using an Aimbot?"
With tracers on it's easy to see consistant errors (Drat! I ALWAYS miss behind my target!) and adjust to correct the problem. With them off, you'll see that you miss, but have no idea where you're shooting... Ghosth's idea of using the Lead Computing Gunsight works because it supplies the needed feedback which is missing w/o the tracers turned on. If you felt there was a benefit to having them off, you could try learning this way.
Learning to shoot is just that, LEARNING. How do you learn with no feedback showing you whether you're on the right track? As a past shooting sports instructor I can say that a "students" target gives a lot of information that can be used to accelerate the learning process. That's because we can see the misses (non bullseye hits) and form strategies to turn them into hits (bullseye's). If I suspended a golf ball 20 feet off the ground and let my students shoot at that, it would be much more difficult to see patterns that lead to misses and to correct them. Of course, if I let them use tracer rounds to shoot at that suspended ball I could say "Look! See that? You're shooting low, aim a bit higher!"
Tracer round ammo doesn't have the same ballistics as the "normal" rounds? So what. It still gives us a "ballpark" answer. At least it's a "hint". Even when we teach people to shoot skeet that "ballpark" answer is enough to go by. Having an experienced shooter watch over your shoulder supplies feedback, or even watching the wad gives a basic answer even though its flight is obviously different than the shot column. Seeing the wad fly consistantly behind the clay target gives a hint that you may need to increase your lead.
Without feedback of some sort we're left with the old standby- "Use the Force, Luke!"